This invention relates to a liquid crystal display device using a dichroic dye.
As a display for information processing equipment such as a personal computer, a liquid crystal display device (LCD) of TN (twisted nematic) mode or STN (super twisted nematic) mode is extensively utilized at present.
In TN or STN-LCD, the alignment of liquid crystal molecules in the liquid crystal cell is kept twisted in the initial state. Light entering into the liquid crystal cell emerges from the cell accompanying with a change in the polarized state that has been induced by the twisted alignment and birefringence of the liquid crystal molecules. On the other hand, when an electric field is applied to the liquid crystal cell, the liquid crystal molecules are realigned in the direction of electric field so that the twisted structure thereof is released and the birefringence is no more generated, and hence the incident light emerges from the cell without any change in polarized state. Therefore, when a liquid crystal cell is sandwiched by a pair of linear polarizers, the change in optical characteristics of liquid crystal layer due to the application of electric field can be observed as a change in intensity of light.
The LCD is advantageous over the CRT (cathode ray tube) display because of its low power consumption. However, since the polarizers are employed in the TN- or STN-LCD, the utilization efficiency of incident light on the LCD is substantially low. Therefore, in order to ensure a sufficient brightness, most of the LCD is provided with a backlight. In particular, in an LCD provided with a color filter, a relatively strong backlight is required, because the light transmitting through the liquid crystal will be considerably reduced. The power required for the backlight is almost equal to the power consumption required for driving the liquid crystal cell, the backlight becomes a cause for limiting the operating time in a potable display in which the electric power is supplied by a battery. Moreover, the backlight is not preferable in the respect that it would lead to the fatigue of operator's eyes.
Under the circumstances, the development of a reflective liquid crystal display device of guest-host mode (GH-LCD) in which the backlight is not necessary has been extensively studied. The GH-LCD employs a liquid crystal containing a dichroic dye and is advantageous in that it enables to obtain a bright color display and is relatively wide in angle of visibility.
As for the driving system of LCD, driving by a thin film transistor (TFT) is now getting more popular because of the advantages that it is thin and light and enables to obtain a clear image. In the TFT driving system, a fluorinated liquid crystal can be employed more advantageously because it is highly hydrophobic.
In order to realize a bright and clear color display in the GH-LCD, the liquid crystal is required to contain a large quantity of dichroic dye so as to represent high absorbance. Accordingly, the dichroic dye is required to have high solubility and dissolution stability in relative to the liquid crystal (in particular, to a fluorinated liquid crystal). Note that, if the cell gap is enlarged so as to compensate a low absorbance, the power consumption would be increased and the response of the liquid crystal to the electric field would be also deteriorated.
Further, in order to realize a GH-LCD driven by TFT, it is very important that the liquid crystal has a high voltage holding capacity. In order to ensure this property, the resistance of the liquid crystal layer is required to be sufficiently high. However, there is a problem in this case that the resistance as well as the voltage holding capacity of the liquid crystal layer may be extremely deteriorated due to a photo-degradation of the dichroic dye mixed in the liquid crystal. Therefore, the dichroic dye is desired to be excellent in resistance to photo-degradation.
At present, an azo dye or an anthraquinone dye has been mainly employed as a dichroic dye. The azo dye is excellent in linearity of molecule, and thus most azo dyes are excellent in solubility to a liquid crystal having a linear structure. However, the azo dye is defective in that it is poor in resistance to light. On the other hand, the anthraquinone dye is excellent in resistivity to light as compared with the azo dye and hence deemed to be more suited for use in the TFT driving system. However, the conventional anthraquinone dyes are poor in solubility to the liquid crystal.
Meanwhile, there is also known a coumarin dye as a dichroic dye having a high absorption coefficient. However, the conventional coumarin dyes are hardly soluble to the liquid crystal at low temperatures. Therefore, when the LCD is exposed to a low temperature, the coumarin dye is caused to precipitate. Once the coumarin dye is precipitated, it is very difficult to redissolve the coumarin dye into the liquid crystal.
Under the circumstances as mentioned above, many attempts have been made to improve the solubility of anthraquinone dyes and coumarin dyes. However, none of them have been successful to sufficiently improve the solubility of these dyes.